Cognos Bursting question (Cognos 10)

I have a burst report with Burst Key defined as Location and burst recipients are email addresses. The issue is that two burst keys may have the same email recipient, in which case the recipient should receive one email instead of two (with a list report showing both Locations).
(I have over 100 Locations, and one person may be handling 25 locations, and I don;t want him to receive 25 emails).

I'm not sure how to use an email address as the burst key. Here are some more details on my report:
Package has two data sources - one's a cube with Location and revenues and the other is an Excel file with Location and email address. My burst key is the Location from the cube source. I can't change that burst key to the email address, because (1) the revenues are not related to the email address and (2) the report gives an error when I include a query item from external data to the cube mixing relational and dimensional).